Hydraulic actuators in many machines are subjected to varying loads, including overrunning loads and resistive loads. An overrunning load (also referred to as an aiding load) is a load that acts in the same direction as the motion of the actuator. Examples of overrunning loads include lowering a wheel loader boom or lowering an excavator boom, each with gravity assistance. A resistive load is a load that acts in the opposite direction as the motion of the actuator. Examples of resistive loads include raising a wheel loader boom or raising an excavator boom, each against the force of gravity.
In certain applications, hydraulic actuators can be subjected to both an overrunning load and a resistive load in the same extend or retract stroke. For example, and with reference to FIG. 1, an exemplary excavator linkage is shown whereby the arm function is positioned in three different positions:
a.) Arm in “neutral” position, cylinder roughly at half displacement;
b.) Arm in “out” position, cylinder retracted; and
c.) Arm in “in” position, cylinder extended.
When an excavator arm actuator that is fully retracted (arm linkage “out”) is given an extension command (arm linkage curling “in”), the motion starts with an overrunning load and then switches to resistive load due to the linkage configuration. The arm actuator in this case is said to have gone “over-center”. The same holds true when the actuator is retracted and goes from an overrunning load to a resistive load as the arm linkage moves outward. The transition between the resistive load and the overrunning load without a change in the direction of motion is referred to herein as an “over-center load condition.” An over-center load condition may occur during a transition from a resistive load to an overrunning load and during a transition from an overrunning load to a resistive load.
In existing hydraulic control systems using spool valves, pressurized hydraulic fluid is supplied from a pump to the cylinder (actuator) and hydraulic fluid flows out of the actuator to a tank. The flow of hydraulic fluid to the actuator and out of the actuator is controlled by a spool, the flow direction being dictated by a position of the spool. The design of a four way spool valve is such that a given position of the spool determines the “flow in” and the “flow out” restriction sizes. Thus, metering in and metering-out are coupled, where a certain restriction size on the inlet corresponds to a certain restriction size on the outlet. Therefore, it is a one degree of freedom system and, as a result, only one of the speed or the hydraulic force can be independently controlled. Such limitation can make it challenging to properly control the desired actuator behavior when transitioning between a resistive load and an overrunning load (i.e., an over-center load condition).
For example, it is desirable that an over-center load condition not affect the velocity of retraction or extension of the actuator. Such velocity control is particularly difficult when the hydraulic actuator is an unbalanced actuator of an electro-hydraulic actuation (EHA) system. An EHA system is a system in which a reversible, variable speed electric motor is connected to a hydraulic pump, generally fixed displacement, for providing fluid to an actuator for controlling motion of the actuator. An unbalanced actuator has unequal cross-sectional areas on opposite sides of the piston, generally as a result of a rod being attached to only one side of the piston. Due to the unbalanced nature of the actuator, as the system transitions into an over-center condition, a speed change occurs in the actuator motion due to the unequal cross-sectional area between the head-side and rod-side of the actuator. Such change in speed is undesirable, as it is difficult for a user to predict when the change will occur and thus can make it difficult to precisely position the working machine during the over-center event.
Further, spools are typically designed such that the outlet is restricted to limit fluid flow and prevent a load from falling at uncontrollable speeds in the event of an overrunning load. However, in other operating conditions, such as lifting the load, such restriction is not needed yet it is inherent in the design of the spool valve. This causes undesired energy loss.